blood brother vs ally

blood brother and ally both are nouns.

Word	Noun	Adjective	Verb	Adverb
blood brother	Yes	No	No	No
ally	Yes	No	Yes	No

As nouns, ally is a hypernym of blood brother; that is, ally is a word with a broader meaning than blood brother:

blood brother: a male sworn (usually by a ceremony involving the mingling of blood) to treat another as his brother
ally: an associate who provides cooperation or assistance

Other hypernyms of blood brother include friend.

blood brother (noun)	ally (noun)
a male with the same parents as someone else	a friendly nation
a male sworn (usually by a ceremony involving the mingling of blood) to treat another as his brother	an associate who provides cooperation or assistance

blood brother (verb)	ally (verb)
	become an ally or associate, as by a treaty or marriage